Microbial infestation of surfaces relies on mechanisms where the microbe establishes adhesion to the surface, where this is a first step in the proliferation of the organism. Inhibition of the initial adhesion step is a potential route to minimising biofouling of surfaces.

Approach

As the existing antifouling technologies relied on the incorporation of biocidal compounds, there are ongoing concerns over the potential development of biocide resistant pathogenic organisms. The need for greener technologies has driven the researchers to develop environmentally benign coatings, such as antifouling surfaces with hydrophilic properties which can resist attack from a range of microbes through a mechanism involving a strongly bonded hydration layer acting as a natural barrier.

From our previous work a commercially available silica nano particle (SiNP) modified with the epoxy silane – glycidyloxypropyltrimethoxysilane (GPS) was fabricated as a coating which demonstrated significant resistance against various proteins, bacteria and fungal spores. As a benign coating, it is economically viable, scalable, and can be easily incorporated into existing or next-generation fabrication processes.

This study aims to elucidate the mechanisms underlying the antifouling performance of GPS-SiNP. 3D Frequency Modulation Atomic Force Microscopy (3D FM-AFM) was applied to resolve the 3D interfacial water structure above single nanoparticles with angstrom resolution, with the observations validated by all-atom MD simulation. Furthermore, Single Cell Force Spectroscopy was used to directly probe the interaction between living fungal spores and the surfaces.

Outcomes

We experimentally probed the hydration structure on single nano silica surfaces at a sub-angstrom level, building up a 3D interfacial structure from which 1D and 2D structural information was derived. The results of MD simulations showed good agreement with the experimental work.

The differences between unmodified SiNP and GPS-SiNP surfaces were compared. For the SiNP surface, the interfacial structure was discrete, extending to a distance of one or two layers of water molecules. However, the existence of energy minima may increase protein residence times and enable amino acid residues to perturb water layers and undergo binding to the substrate.

In contrast, with the flexible hydrophilic GPS chains associated with the water molecules, the confluent GPS-water layer present on the GPS-SiNP surface extended a much greater distance with a significantly higher repulsive force. This layer acted as a barrier and provided a basis for developing a broad-ranging fouling resistance system, from protein adsorption to microbial adhesion of bacteria and fungal spores.

The combination of the 3D FM-AFM measurements and the MD simulation helped to elucidate the molecular-level interfacial structures and to explain the exceptional antifouling ability of useful chemistries and coating.

Single Cell Force Spectroscopy was used to study the interaction between the two proposed surfaces and fungal spores. It was found that, compared to the SiNP surface, the GPS-modified SiNP surface showed far less adhesion behaviour and much smaller adhesion force. This correlated well with previous results. This finding gives unprecedented insight into the mechanism by which a ‘real’ living fungal spore interacts and adheres to a surface, e.g. critical information for the design of high-performance antifouling coatings.

Future work

A better understanding of the relationship between hydration structure and antifouling performance, coatings with other chemistries is to be obtained using both 3D FM-AFM and Single Cell Force Spectroscopy.